Strains/fiber elongation, forming

Once an object is formed, the internal stresses which result from cooling are usually reduced by annealing. The annealing point (cited in various sources as either 10 or 10 " Pa s), which is also determined using a fiber elongation test, is defined as the temperature where the stress is substantially relieved in a few minutes. The strain point (1013.5 ig defined as the temperature where stress is substantially relieved in several hours. The strain point is determined by extrapolation of data from annealing point studies. Other tests are also used for these two reference points, with slightly different results. 

As far as ASME Code is concerned, there is no difference between fiber elongation, FE, and forming strain, f. In ASME VIII-1, it is called fiber elongation. In ASME VIII-2, it is called forming strain. In reality, the only way to determine actual fiber elongation is to remove a sample section of material, macroetch the sample, and view under a microscope. 

The ratio of stress to strain in the initial linear portion of the stress—strain curve indicates the abiUty of a material to resist deformation and return to its original form. This modulus of elasticity, or Young s modulus, is related to many of the mechanical performance characteristics of textile products. The modulus of elasticity can be affected by drawing, ie, elongating the fiber environment, ie, wet or dry, temperature or other procedures. Values for commercial acetate and triacetate fibers are generally in the 2.2—4.0 N/tex (25—45 gf/den) range. 

As expected, the residual extensibility of the fiber decreases at higher draw ratios. What is not so predictable is that the true stress at failure increases as the draw ratio increases fiber failure strength is improved by drawing the yarn. If a curve is drawn to connect the end points of the stress-strain curves, it is seen that there is an inverse relationship between tenacity and elongation to break (eb). The form of this relationship is as follows ... 

Condensation polymers tend to exist below their Tg at room temperature. They typically form fairly ordered structures with lots of strong interactions between the various chains giving strong materials with some, but not much, elongation when stretched. They are normally used as fibers and plastics. They have high stress/strain ratios. 

The effect of the HRH system on adhesion is further illustrated by the micrographs (Figures 7-11) of the same rayon-natural rubber composite with and without HRH. Figures 7-9 show a thin section of the composite without HRH stretched to various elongations with the force applied parallel to the direction of orientation. Many voids form as the strain is increased owing to fiber-matrix bond failures. Both the number and size of voids increase with increasing strain. 

Figure 7a and b show typical three-dimensional surface topographic images of the PHB fibers drawn at a draw ratio of 4.0 and 7. The surfaces of the fibers differ considerably. Depending on the draw ratio, spherulitic or fibril-like surface structures were formed. The textile physical properties of the fibers can be explained by these different structures. The fibers, spun at a draw ratio of 4.0, are brittle without a sufficient elongation at break visible in the stress-strain curve (Fig. 5). The fibers spun at a draw ratio of 7 show a completely different stress-strain behavior with a sufficient elongation at break and a sufficient tenacity, as can be seen from the stress-strain curve (Fig. 5).

The production of crystals in a polymer by the action of stress, usually in the form of an elongation. It occurs in fiber spinning, or during rubber elongation, and is responsible for enhanced mechanical properties. Simultaneous readings of load and deformation, converted to stress and strain, plotted as ordinates and abscissas, respectively, to obtain a stress-strain diagram. 

The model was also extended to describe the composition dependence of the tensile strength of heterogeneous polymer systems, including particulate filled and fiber reinforced polymers, as well as blends [6, 35, 36]. During the elongation of the specimen the cross-section of the specimen decreases continuously and orientation leads to strain hardening. These effeets have to be taken into account thus the modified eorre-lation takes the form ... 

Tenacity (stress) and elongation (strain) together with elastic modulus are the most important mechanical properties of yarns. For staple fiber yarns, they depend on the twist level. For filament yarns, they can be set in the drafting zone by orienting the macromolecules accordingly and by forming crystalline and amorphous regions. 

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